1. Field of the Invention
The present invention relates to a process for producing highly pure terephthalic acid. Particularly, it relates to a process for producing highly pure terephthalic acid by suppressing formation of impurities at the time of purifying crude terephthalic acid obtained by an oxidation reaction of p-xylene, by contacting it with hydrogen in the presence of a platinum group metal.
2. Discussion of the Background
Crude terephthalic acid obtained by oxidation of p-xylene usually contains relatively large amounts of various impurities including 4-carboxybenzaldehyde (hereinafter referred to simply as "4CBA"). Heretofore, it has been common that such terephthalic acid is purified and then used as a starting material for polyester.
As a purification method for such crude terephthalic acid, a method of subjecting the crude terephthalic acid to hydrogenation treatment or oxidation treatment has been known. As a method of subjecting crude terephthalic acid to hydrogenation treatment, a method has been proposed wherein an aqueous solution of crude terephthalic acid is subjected to hydrogenation reduction treatment at a high temperature under high pressure in the presence of a hydrogenation catalyst, and crystals of terephthalic acid are recovered from the treated aqueous solution (Japanese Examined Patent Publication No. 16860/1966).
By such a method of purifying crude terephthalic acid by hydrogenation, it is possible to remarkably reduce metal ions as inorganic impurities in the crude terephthalic acid and the amount of 4CBA which constitutes the largest content among organic impurities. However, by this method, it is impossible to completely remove organic impurities having unknown structures (hereinafter referred to as "coloring impurities") other than 4CBA, and part of such coloring impurities will remain in the purified terephthalic acid and will bring about, for example, coloring during the production of polyester, whereby whiteness of the polymer tends to deteriorate.